1. Field of the Invention
The present invention relates to a method and an apparatus for calling a handset unit in a radio telephone system, and more specifically to a method and an apparatus for calling or reaching a handset unit belonging to a radio telephone system of a composite radio telephone system from a telephone set belonging to the same radio telephone system, the composite radio telephone system being comprised of a plurality of radio telephone systems connected through communication circuits to one other, in which a handset unit belonging to one of the radio telephone systems can be used in other radio telephone system.
2. Description of the Related Art
The above mentioned composite radio telephone system is, for example, a cordless telephone system of the second generation, which is under review among Japanese government circles at present. In the cordless telephone system of the second generation, a plurality of public radio base stations and a control center, which controls communications through these public radio base stations, are connected to a communication network which has already been in service, such as a Public Switched Telephone Network (PSTN) and an Integrated Service Digital Network (ISDN). The cordless telephone system allows a handset unit of a cordless telephone apparatus for domestic and business use to be used outside in a similar manner to a conventional Cellular Phone as now being used, when the handset unit has been registered in the control center. That is, the cordless telephone system of the second generation allows a user to use a handset unit not only in a private area, such as at home and at a business place, but also anywhere else for transmitting his voice and receiving voice of a person on the other end.
In the cordless telephone system of the second generation, there are various methods of calling or reaching the handset unit registered in the control center from another telephone set of the cordless telephone apparatus. Routes or channels to reach the handset unit are different depending on places where the handset unit is used, that is, depending on whether the handset unit is used in a private house or in a business place, or whether the handset unit is used outside. In other words, when used in the private house or in the business place, the handset unit can be reached through a private radio base station that is installed in the private house or in the business place, in a similar manner in which a conventional cordless telephone apparatus is reached. A user of a telephone set, therefore, dials for calling an extension phone, i.e., he (or she) operates an extension selection key, and then dials the extension number. Meanwhile, when the handset unit is used outside, the user of the telephone set will dial to get an outside line since the telephone set reaches the handset unit on the other end through the communication network and the public radio base station.
As the handset unit of the cordless telephone apparatus, however, can be used outside in the cordless telephone system of the second generation, the user often takes out the handset unit without telling anyone of his family or fellows. In this case, a person who wants him on the phone operate the extension key at first to reach him at home or at the business place, and when he (or she) is told the user was out, then he has to dial again to get an outside line. As described above, there is a drawback that requires users of the handset units to operate complex dialing operation.
A similar drawback will be invited not only in the cordless telephone system of the second generation but in another composite radio telephone system such as a system, in which a plurality of independent cordless telephone apparatus are installed respectively in separate business places, and are connected to one other through leased circuits, and in which a handset unit of the cordless telephone apparatus installed in one business place is capable of communicating with a radio base station of the other cordless telephone apparatus in other business place.
The present invention has been made to overcome the above mentioned drawback, and has an object to provide a method and an apparatus, for calling or reaching a handset unit, with a simple dialing operation, belonging to a radio telephone system of a composite radio telephone system from a telephone set belonging to the same radio telephone system.